Vibration caused by mass imbalance is a common problem in rotating machinery. Imbalance occurs if the principal axis of inertia of the rotor or other rotating element is not coincident with its geometric axis. The higher the rotational speeds, the greater the centrifugal imbalance forces.
There are generally two types of systems which have been employed to reduce vibration in rotating machinery, namely, passive systems and active systems. Although an active control system is usually more complex than a passive control scheme, it has a number of advantages. First, active vibration control is generally more effective than passive control. Further, passive vibration control is of limited use if several vibration modes are excited. Additionally, because active vibration control devices can be adjusted according to vibration characteristics during operation of the machinery, active vibration techniques are much more flexible than passive vibration control.
Two major categories of active vibration control techniques for rotating machinery have been used in the past. These include direct active vibration control (DAVC) and active balancing. DAVC techniques directly apply a lateral control force to the rotor which is generated by a force actuator such as a magnetic bearing. One advantage of DAVC techniques is that the input control force to the system can be changed quickly. By applying a fast-changing lateral force to the rotating machinery, the total vibration, including synchronous vibration, transient-free vibration and other non-synchronous vibration, can be suppressed. However, DAVC systems are often limited by the maximum force which can be applied by the lateral force actuators employed. At high rotating speeds, the imbalance-induced forces can reach levels beyond the compensation capability of the force actuators in DAVC systems.
Active balancing techniques are typically used in high rotating speed situations where DAVC systems are inadequate. In active balancing schemes, a mass redistribution actuator, e.g. a device whose mass center can be changed, is mounted on the rotor or other rotating element. Initially, the vibration of the rotating system is measured and the imbalance of the rotating machinery estimated, and then the mass center of the actuator is changed to offset the system imbalance. The vibration of the rotating machinery is suppressed by eliminating the root cause of the vibration—system imbalance.
Active balancing systems have an advantage over DAVC systems in that they can provide large compensating forces. However, in systems currently available, operation of the mass redistribution actuator is typically relatively slow. Additionally, while most active systems can eliminate imbalance-induced synchronous vibration, difficulties arise in suppressing transient vibration and other non-synchronous vibration.